1. Field of the Invention
The present invention is related to a gain structure of a stretchable antenna for mobile phones, and especially to a gain structure of a stretchable rod antenna capable of increasing capability of signal receiving and emitting in cooperation with a coil antenna when the rod antenna is stretched.
2. Description of the Prior Art
A conventional antenna for mobile phones is divided mainly into two kinds; they are the quarter-wave and the half-wave antennas. The quarter-wave antennas have lower impedance in servo linking, and do not need using of impedance transformers, but they are inferior to the half-wave antennas in the capability of signal receiving and emitting. The half-wave antennas have higher impedance, and need using of impedance transformers in the interior of their mobile phones.
In the recent years, mobile phones tend to be minimized. Half-wave antennas have higher impedance and need impedance transformers which occupy larger space in their mobile phones; other redundant parts used further increase their costs. Lengths of half-wave antennas generally are hard to suit modern pocket mobile phones; such lengths are disproportional to the main bodies after assembling. In view of this, pocket mobile phones generally use quarter-wave antennas or quarter-wave coil antenna; however, quarter- wave antennas are evidently insufficient in the capability of signal receiving and emitting even when the rods of antennas as radiators are fully stretched.
In a U.S. patent "Antenna device for portable equipment" U.S. Pat. No. 5,757,325 granted to Saldell, an impedance transformer is evidently used.
In another U.S. Pat. No. 5,661,495 granted to Saldell, a technique is disclosed to use a switching device to separate a quarter-wave antenna when the antenna is retracted; however, it is still insufficient in the capability of signal receiving and emitting.
In another U.S. Pat. No. 5,771,023, a coil antenna for broad bands is disclosed, wherein, a broadband antenna is formed by connecting two coil antennas respectively with a rod antenna.
A U.S. Pat. No. 5,650,789 provides a retractable antenna for an indoor wireless telephone set, of which two coil antennas are respectively used at the end of a rod antenna and the telephone set. The coil antenna at the end of the rod antenna is electrically broken when the latter is stretched out, but can make a fixed coil antenna on the surface of an engaging seat form electromagnetic coupling with the coil antenna at the end of the rod antenna for signal receiving and emitting when the rod antenna is retracted. This patent still cannot increase capability of signal receiving and emitting by using a quarter-wave antenna.